Subsequent studies showed no change in cortisol levels in men. In the latter study, subjects had a 15-day pre-bed-rest period during which they were permitted to adapt to the protocol and environment. Similarly, spaceflight studies of humans showed unchanged cortisol levels after an initial adjustment period. Table 1 summarizes the results of studies of male reproduction utilizing ground-based models of simulated hypogravity. In humans, 6° head-down tilt bed rest for 60-120 days altered sperm morphology and reduced the number of active spermatozoa. This finding has important implications because in humans even slight reductions in sperm numbers compromise reproductive capacity.
Bed rest had no effect on plasma T or LH but did produce a reduction in plasma FSH and prolactin levels. Both FSH and prolactin are involved in the regulation of spermatogenesis, suggesting that reductions in sperm number may have been due to simulated hypogravity-induced changes in endocrine function.
Rats exposed to simulated hypogravity using the Morey-Holton HLS model exhibited increased cortisol during initial HLS. No effects have been reported on stress-inducible transcripts or on adrenal gland weights, suggesting the absence of stress in HLS animals following initial adaptation. Congruent with the spaceflight studies, HLS rats had reduced testes weights. Longer-term (>14 days) HLS of male rats resulted in >50% reductions in testes weight compared with nonsuspended controls.